The surfaces of downhole tools, when in contact with an abrasive environment such as a borehole wall, can undergo a high level of abrasion. In light of this, these surfaces are oftentimes coated with an abrasive resistant coating, in an effort to reduce wear and extend tool life. For example, abrasive resistant coatings, or hard facings, are often applied to susceptible areas of a tool such as wear bands, directional drilling pressure pads and stabilizers. Coatings such as these are typically a particulate metal matrix composite, based on a nickel or cobalt alloyed matrix containing tungsten or titanium carbides particles. Using such a combination, both high degrees of hardness and toughness can be obtained.
These coatings are traditionally applied using a variety of methods such as weld overlays (MIG, plasma transfer arc, laser-cladding), thermal spray processes (high velocity oxygen fuel, D-gun, plasma spray, amorphous metal) and brazing (spray and fuse techniques) as know by those skilled in the art. In addition, wear resistant inserts, such as cemented tungsten carbide tiles or polycrystalline diamond (PDC, TCP) inserts are often attached to critical areas by brazing or other means to increase the wear resistance. Existing abrasive resistant coatings such as these result in the application of a coating over a substrate that has a non-uniform surface that is oftentimes rough in texture.
While numerous abrasive resistant coatings have been produced for wear-resistant applications, none have been specifically designed to withstand the harsh environmental conditions encountered in downhole environments. The rubbing of a metal against a rock formation in the presence of drilling mud under high stress, together with repeated impact loading, creates a unique set of mechanisms that can lead to very rapid material loss.
In such an environment, the abrasive wear exhibited by traditional abrasive resistant coatings can be divided into two categories, namely brittle wear and ductile wear. Brittle wear occurs due to cracking and material removal at the surface of the abrasive resistant coating while ductile wear is exhibited by gradual material removal which results in a smoothing effect on the surface. In contrast, ductile wear is described by a slow smoothing of the ductile component of the matrix material. Ductile wear in an abrasive resistant coating increases when more of the ductile components of the abrasive resistant coating are exposed to the abrasive environment. The extent by which an abrasive resistant coating exhibits brittle or ductile wear is dependent on the local load the material must bear while in operation as well as the individual components exposed to the abrasive environment. For example, if the material at the surface of the abrasive resistant coating is brittle and the load applied is higher than its fracture stress (fracture under compressive load), the wear mechanism is brittle. In the alternative, if the load applied to the abrasive resistant coating is less than the fracture stress of the abrasive resistant coating, material is removed by a ductile wear mechanism. The wear rate under brittle wear is significantly higher than that in ductile wear. See I. M. Hutchings, Tribology: Friction and Wear of Engineering Materials, 1992 (incorporated herein by reference in its entirety).
Existing approaches to minimizing wear in an abrasive resistant coating have resulted in the increase of the bulk hardness of the abrasive resistant coating by increasing the fraction of tungsten carbide reinforcement used in the abrasive resistant coating. Such an increase in the carbide volume fraction results in an increase of the wear resistance. However, at very high carbide volume fractions, extensive cracking can occur, as insufficient ductile matrix material is present to accommodate the residual stresses created during processing. For example, an abrasive resistant coating with a high carbide volume fraction applied using a plasma transfer arc method will likely result in a non-uniform surface that exhibits excessive cracking at various regions due to the lack of sufficient ductile matrix material. In the alternative, an abrasive resistant coating with a high percentage of exposed ductile material will undergoes rapid wear of the ductile matrix material, resulting in decreased abrasive resistant coating life.
In view of the above, a system, method and apparatus which results in the reduction of abrasive wear in abrasive resistant coatings is needed.